This invention relates generally to continuous profile molding methods and products. More particularly, the present invention relates to processes for manufacturing composite rod assemblies that may be used, for example, as tool handles, having a construction which significantly increases the strength of such rod assemblies without a significant corresponding increase in weight.
Continuous profile molding is generally known as pultrusion in the thermosetting field and extrusion in the thermoplastic field. The process of pultrusion involves the manufacture of articles having a continuous profile of a single selected cross-section matching that of a die. Usually the manufactured article comprises a thermosetting type resin (i.e., polyesters, epoxies, phenolics, etc.), reinforced with such materials as glass fibers, Boron, Kevlar, hemp, cotton, sisal, etc. Pultrusion manufacturing processes have a tremendous number of applications, but there is also a significant limitation, i.e., the articles produced have only one continuous profile (round, square, hollow, channel, etc.) in cross-section. The only modification to that cross-section will occur after the product has exited from the die by such means as grinding, cutting, drilling and/or sawing, all of which add substantial cost to the end product as well as generally degrading the initial pultrusion or molding as it pertains to its physical properties. To modify a cross-section by one of the above mentioned operations, one must remove by grinding, drilling, etc., some of the material already used to produce the original product. In doing so the reinforcing material (glass fiber) is cut, ruptured or abraided. Thus, several undesirable things take place: (1) cost for labor and/or equipment are increased; (2) there is a degradation of the finished product; and (3) raw material originally used to make the full cross-section or profile of the article is wasted. An additional and usually more expensive manner by which the cross-section or profile of the pultruded article can be modified is to mold or machine additional collars or adapters of plastic or metal materials which can then be attached to the original cross-section profile by bonding, riveting, screwing or otherwise attaching the collar or adapter to provide a different cross-section in a desired location.
In recent years pultrusion manufacturing processes have been adapted to manufacture composite rod assemblies that may be used as handles for hand tools such as a shovels, rakes, hoes and the like. The basic technique for running filaments through a resin bath and then through an elongated heated die tube to produce a cured composite rod of the same shape as the die tube has been known for some time. See, for example, U.S. Pat. Nos. 2,948,649 and 3,556,888. This method, however, produces a solid extruded product which is unacceptably heavy and/or too rigid for many tool handle applications. The weight problem can be alleviated by means of an existing process to extrude hollow tubes utilizing a die tube with the center filled, leaving an annular cross-section through which the resin coated fibers are pulled. This weight reduction is achieved, however, at the cost of significantly reduced bending or flexural strength in comparison with a solid rod, resulting in a tool handle which would not be suitable for use in certain high-stress applications such as general purpose shovel handles. Further, to increase interlaminar strength of the tube forming fibers, a substantial percentage of fibers running other than in a longitudinal direction have been thought to be required.
The bending strength of tool handles can be improved by producing fiber-resin rods which are substantially hollow or lightweight throughout a major portion of their length, but reinforced at areas of expected high stresses during tool use. Such improved tool handles and related methods are shown in U.S. Pat. No. 4,570,988, the contents of which are incorporated herein by reference. These composite tool handles have further been improved by the introduction of one or more reinforcing beads of fiber-resin material extending the length of the load-bearing rod. Such tool handles are shown in U.S. Pat. No. 4,605,254, the contents of which are incorporated herein by reference.
Although such above-described composite tool handles are generally superior to wooden handles, the competitive pressures of the marketplace have encouraged tool handle manufacturers to seek new processes, materials and construction techniques to further increase the strength of composite tool handles without introducing additional weight and/or cost to the handle. In this regard, it is important to permit use of the most economical glass fibers and the most reasonably priced resins to produce a product that has the greatest value to the end user. However, common glass fibers and resins have physical properties which are often less desirable when utilized in a composite tool handle than other more exotic and costly fibers and resins. Accordingly, one objective is to obtain higher mechanical strength properties in a composite material tool handle while permitting the manufacturer to use relatively less costly materials.
It is well known that utilizing unidirectional strands of resin coated glass fibers in a pultrusion process is the most economical process for manufacturing a composite rod assembly. In many cases glass fibers such as a fabric mat veil have been introduced into the pultrusion process to reduce interlaminar failure or to increase the hoop strength of the rod assembly by providing cross-fibers within the cured fiber-resin composite load-bearing jacket. The use of cross-fibers, however, typically and undesirably increases the costs associated with manufacture of composite rod assemblies and decreases tensile strength along the length thereof. Thus, to increase interlaminar and hoop strength of the composite rod assembly, some tensile and flexural strength is sacrificed.
Stress testing of composite rod assemblies has revealed several common characteristics as they fail under increasing loads. When a flexure load is applied perpendicularly to the longitudinal axis of a composite rod assembly, the first failure usually occurs very close to the center of mass perpendicular to the applied load and extending longitudinally through the rod assembly. This failure is in shear, between the fibers of the resin. Following this initial shear failure, the rod assembly is then separated into two relatively equal half sections which perform as independent units at half the overall load-bearing value of the original composite rod assembly. As the load is increased further, the next failure occurs as a compression failure in the bottom half of the original section of the rod assembly. Composite rod assemblies are far stronger in tension (due to the strength characteristics of the fiber materials), whereas the compressive loads are borne almost entirely by the interfiber resinous material.
One method of reducing the shear failure problem noted above is to mold a cladding material over at least a portion of the outer surface of the manufactured rod assembly where the greatest stresses are likely to occur. It is often desirable, however, to mold a thermoplastic cladding material over the composite rod assembly which, due to the inherent physical properties of commonly used thermoplastic materials, can present additional manufacturing difficulties. In this regard, composite rod assemblies utilizing fiberglass as a strength member have a perfect elastic memory which causes the rod assembly to return to its normal shape after an infinite number of flexes. Further, within its limits, fiberglass has virtually no cold flow. When a thermoplastic molding is provided over a composite rod, the thermoplastic material provides the desired cosmetic look and the bulk to properly fit a user's hand. Use of thermoplastic materials alone to manufacture many types of composite rod assemblies, including tool handles, is not desirable because of the thermoplastic material's inherent low strength.
There is a wide range of thermoplastics available for use as a cladding over composite rod assemblies. These materials range in price from a few pennies per pound to several dollars per pound. Under circumstances where the physical properties of the thermoplastic material are not critical when molded over a composite rod assembly, it is, therefore, desirable to find the most economical material to produce the desired article. When a product such as a sledge handle or a shovel handle 36 inches to 48 inches long has a relatively heavy thermoplastic cladding molded over it, shrinkage occurs that can be as high as 0.010 to 0.015 inch per inch. Thus, if the cladding completely encapsulated the composite rod assembly, the shrinkage would be of such magnitude that the stresses within the cladding material will exceed its tensile strength and simply break apart.
Accordingly, there has been an on-going need for improved composite rod assemblies and related manufacturing processes to provide significantly increased tensile and flexural strength without a corresponding increase in the weight thereof. Such a manufacturing process preferably permits use of relative low-cost fiber and resin materials, and utilizes unidirectional fibers in a pultrusion manufacturing process. Additionally, there exists a need for a composite rod assembly having increased interlaminar and hoop strength without the use of cross-fibers. Further, a manufacturing process is needed which is compatible with prior techniques for localized strengthening of composite rod assemblies, as by, for example, the use of alternating sections of lightweight filler core and strong reinforcing core within a composite load-bearing jacket, and the use of longitudinally extending reinforcing beads. Moreover, a novel composite rod assembly is needed which has greatly improved resistance to shear failure through the resin, as exhibited in prior composite rod assemblies, and which can accommodate use of relatively inexpensive thermoplastic materials. The present invention fulfills these needs and provides other related advantages.